Autopilot-based steering and maneuvering system for boats

ABSTRACT

A boat featuring an autopilot-based steering and maneuvering system. The steering system uses a specially integrated autopilot that remains engaged unless the operator is actively commanding the boat to change course. For example, in a boat in which steering is performed using a joystick, course changes can be effected simply by moving (e.g., twisting) the joystick. That movement automatically disengages the autopilot, allowing the operator to achieve the course change. When the operator has completed the course change and released the joystick, a centering spring returns it to a neutral position and the autopilot automatically reengages. In the improved maneuvering system, the autopilot is used for controlling the direction of a waterjet boat during very low speed (e.g., less than 4 knots) maneuvers, such as docking. The autopilot controls the steering system, e.g., rotation of the waterjet nozzle, to maintain a desired bow direction, while the operator uses a manual control device to apply a sideward force (e.g., from a bowthruster) to move the boat sideways. Preferably, a stick control device (e.g., a multi-axis joy stick) is used, and movement of the stick in a selected direction (sideways, fore and aft, or a combination) causes the boat to move in a corresponding direction, but with the direction of the bow maintained by the autopilot.

BACKGROUND OF THE INVENTION

[0001] The invention relates to steering systems for boats, e.g.,waterjet driven boats.

[0002] Waterjet boats are propelled by drawing a stream of water througha channel in the bottom of the boat and ejecting the stream out the backof the boat. A typical waterjet has two steering components: a nozzleand a reversing bucket. The nozzle is a tubular element near the rear ofthe propulsion stream (“the jet”) that rotates from side to side.Rotating the nozzle deflects the exiting stream, imparting a sidecomponent to the propulsion vector, thereby turning the boat to port(left) or to starboard (right). A nozzle in a waterjet boat essentiallyserves the same purpose as a rudder in a propeller driven boat.

[0003] The reversing bucket allows an operator to slow or back up theboat. The bucket is a curved element located at the aftmost portion ofthe jet, just behind the nozzle. Ordinarily, the bucket is elevatedabove the jet, and has no effect on the operation of the boat. When thebucket is lowered over the jet, it blocks the jet and reverses itsdirection, causing the boat to move backwards. If the bucket is onlypartially lowered, it reverses some of the jet, thereby reducing theforward thrust, but does not reverse the direction of the boat's motion.If the bucket is lowered to reverse approximately half of the jet, thena balance point is achieved, and forward thrust of the boat iseliminated.

[0004] Some waterjet boats also have a third steering element, called abowthruster, for side to side movement at low speed. The bowthruster istypically a tube that runs laterally across the boat near the bow, belowthe waterline. A reversible propeller in the middle of the tube canthrust the boat in either sideways direction.

[0005] Waterjet boats have a number of advantages over traditionalpropeller driven boats, including reduced noise and low draft. Waterjetboats, however, can be notoriously difficult to control, particularly atlow speeds, e.g., when docking. In prior art waterjet boats, maintaininga heading and adjusting course, particularly at very low speed, requiresconsiderable training, especially for operators accustomed totraditional propeller boats.

[0006] To facilitate steering of boats in the open sea, some boatsinclude autopilots. The autopilot, when activated by an operator,maintains the boat's current course. Some propeller boats also include adetent structure to lock in a boat's course. In these boats, thesteering wheel includes a notch or a groove, and the mechanism steeredby the wheel includes a corresponding notch or groove. When the pilotreturns the wheel to a neutral position, the corresponding notch andgroove engage, holding the wheel in the neutral position. In certainboats, the autopilot automatically engages when the pilot returns thewheel to the neutral position and the corresponding notch and grooveengage.

SUMMARY OF THE INVENTION

[0007] We have discovered new ways to use an autopilot to both steer andmaneuver a boat, particularly a waterjet boat.

[0008] In the improved steering system, a specially integrated autopilotremains engaged unless the operator is actively commanding the boat tochange course. The operator need not constantly engage and disengage theautopilot, as is necessary with a conventional system. For example, in aboat in which steering is performed using a joystick, course changes canbe effected simply by moving (e.g., twisting) the joystick. Thatmovement automatically disengages the autopilot, allowing the operatorto achieve the course change. When the operator has completed the coursechange and released the joystick, a centering spring returns it to aneutral position and the autopilot automatically reengages.

[0009] The new steering system is simpler to use than conventionalsystems as the operator does not have to be concerned with manuallydisengaging and then re-engaging the autopilot. The autopilot functionsin the background without the operator ordinarily needing to give it anyattention. The system is also safer, as an instinctive steeringcorrection to avoid an obstacle will immediately disengage theautopilot.

[0010] In the improved maneuvering system, the autopilot is used forcontrolling the direction of a waterjet boat during very low speed(e.g., less than 4 knots) maneuvers, such as docking. The autopilotcontrols the steering system, e.g., rotation of the waterjet nozzle, tomaintain a desired bow direction, while the operator uses a manualcontrol device to apply a sideward force (e.g., from a bowthruster) tomove the boat sideways. Preferably, a stick control device (e.g., amulti-axis joy stick) is used, and movement of the stick in a selecteddirection (sideways, fore and aft, or a combination) causes the boat tomove in a corresponding direction, but with the direction of the bowmaintained by the autopilot.

[0011] This new maneuvering system makes it possible for even a noviceoperator to easily maneuver a waterjet boat in close quarters. Theunsettling effects of wind and tide on the direction of the boat areautomatically compensated for by the autopilot. And the operator is ableto move the boat in and out of a slip, or to and from a dock, simply bymaking intuitive movements of a stick control device.

[0012] In this maneuvering mode, the autopilot's P factor (number ofdegrees of nozzle rotation for each degree of sensed heading error) ispreferably set higher than would be used when the boat is underway. Forexample, P factors greater than 4 (and more preferably greater than 6)have been found to work successfully on a 35 foot Hinckley Picnic Boatpowered by a single waterjet drive.

[0013] A simple and effective implementation of this maneuvering systemis to use a bow thruster to apply sideward force in response to operatormovement of the stick control device. The bow thruster initially changesthe direction of the bow, but the autopilot quickly corrects thedirectional error by producing a compensating rotation of the waterjetnozzle.

[0014] Used in combination, the steering and maneuvering aspects of theinvention make it possible to leave an autopilot constantly on, fromfirst turning on a boat in a slip to driving the boat at high speed onopen water. The new steering system works well in combination with thenew maneuvering system, as if directional changes are desired duringvery low speed maneuvers, the operator simply moves the control devicein the manner required to make a course change (e.g., twisting ajoystick), and then resumes the intuitive maneuvering movements, as theautopilot will then maintain the new boat direction.

[0015] Embodiments of the invention may include one or more of thefollowing features. The boat may be a waterjet boat, e.g., a waterjetboat less than 75 feet in length. The stick control member may beconfigured to rotate to the left and to the right about a generallyvertical axis; rotating the stick control member to the left steers theboat to port, and rotating the stick control member to the right steersthe boat to starboard. The stick control member may be biased to aneutral zero rotation position by a centering torque provided, e.g., bya spring, so that when the operator releases the stick control member,the centering torque returns the stick control member to its neutralposition. The autopilot may be configured to always be engaged when thestick control member is in its neutral position.

[0016] Other features and advantages of the invention will be apparentfrom the following description of the preferred embodiments, and fromthe claims.

BRIEF DESCRIPTION OF THE DRAWING

[0017]FIG. 1A is an elevation view of a prior art boat equipped with awaterjet drive and a bowthruster.

[0018]FIG. 1B is a plan view of the prior art boat of FIG. 1A.

[0019] FIGS. 2A-2C are enlarged, diagrammatic, elevation views of thewaterjet drive of FIG. 1A, showing a reversing bucket in three differentpositions.

[0020] FIGS. 3A-3C are enlarged, diagrammatic, plan views of thewaterjet drive of FIG. 1A, with the reversing bucket in maximum forwardthrust position, and a nozzle in three different positions.

[0021] FIGS. 3D-3F are enlarged, diagrammatic, plan views of thewaterjet drive of FIG. 1A, with the reversing bucket in maximum reversethrust position, and the nozzle in three different positions.

[0022]FIG. 4A is a partially diagrammatic, partially schematic view of ajoystick used for steering the reversing bucket, nozzle, and bowthrusterof the boat of FIG. 1A.

[0023]FIG. 4B is a schematic view of an autopilot used in a preferredembodiment of the invention.

[0024]FIG. 5 is a schematic illustrating communication between thejoystick of FIG. 4A and the autopilot of FIG. 4B.

[0025]FIG. 6 is a schematic illustrating a waterjet boat equipped withan autopilot.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0026] In a preferred embodiment, the invention features a boat having awaterjet drive and bowthruster, a joystick control device, and anautopilot. The autopilot is specially integrated into the boat's controlcircuitry, allowing the autopilot to automatically control the boat'scourse unless the operator is actively commanding a change in course.

[0027] The Waterjet Drive

[0028] Referring to FIGS. 1A and 1B, a boat 10 includes a waterjet drive12 and a bowthruster 16.

[0029] Referring to FIGS. 2A-2C, drive 12 includes an inlet 8, a nozzle18, and a reversing bucket 14. Water jet 20 enters through inlet 8 andexits through nozzle 18.

[0030] FIGS. 2A-2C illustrate the structure and operation of reversingbucket 14. Bucket 14 includes a bucket inlet 22 and a bucket outlet 24.Water from jet 20 which enters bucket inlet 22 is “reversed,” and flowsout bucket outlet 24 in the opposite direction.

[0031]FIG. 2A illustrates bucket 14 in its fully elevated, maximumforward thrust position. In the maximum forward thrust position, bucketinlet 22 remains above jet 20, and does not affect flow of the jet. FIG.2B shows bucket 14 in its neutral position. In the neutral position,approximately half of jet 20 enters bucket inlet 22 and exits bucketoutlet 24 in the reverse direction, such that forward and reverse thrustare approximately equal. FIG. 2C shows bucket 14 in its fully engaged,maximum reverse thrust position. In this reverse thrust position, all ofjet 20 enters bucket inlet 22 and is reversed by bucket 14, causing boat10 to move in reverse.

[0032] FIGS. 3A-3F illustrate the operation of nozzle 18. Rotation ofnozzle 18 in a horizontal plane about a generally vertical axis (notshown) alters the flow direction of exiting jet 20 along the plane ofthe water, changing the “sideways” component of the thrust vector actingon boat 10. Rotation of nozzle 18, therefore, steers boat 10 to port(left) or to starboard (right). A hydraulic pump 68 physically rotatesnozzle 18, in response to commands from a control circuit (FIG. 5).

[0033] FIGS. 3A-3C show nozzle 18 in three different angular positionsfor the case in which reversing bucket 14 is in its fully elevated,maximum forward thrust position. (Bucket 14 does not appear in FIGS.3A-3C because it is elevated above jet 20.) Positioning nozzle 18 asshown in FIG. 3A results in left sideways thrust for boat 10,positioning nozzle 18 as shown in FIG. 3B results in straight movement(zero sideways thrust), and positioning nozzle 18 as shown in FIG. 3Cresults in right sideways thrust.

[0034] FIGS. 3D-3F show nozzle 18 in the same three angular positionsfor the case in which bucket 14 is in its fully engaged, maximum reversethrust position. With bucket 14 and nozzle 18 positioned as shown inFIG. 3D, boat 10 will move in reverse, with a left sideways thrust; withthe bucket 14 and nozzle 18 positioned as shown in FIG. 3E, boat 10 willmove in reverse, with no sideways thrust; and with bucket 14 and nozzle18 positioned as shown in FIG. 3F, boat 10 will move in reverse, with aright sideways thrust.

[0035] The Joystick and Automatic Pilot Controls

[0036] Boat 10 is controlled using a joystick and a specially integratedautopilot.

[0037] Referring to FIG. 4A, a joystick 30 is coupled by electricalcircuitry 31 a, 31 b, and 31 c to bucket 14, bowthruster 16, and nozzle18, respectively. Moving joystick 30 in the forward and reversedirections (the directions of arrows F and B) raises or lowers bucket14, altering the forward or reverse thrust of boat 10. Moving joystickto the left or to the right (in the directions of arrows L and R)engages bowthruster 16, moving boat 10 to the left or the right.Bowthruster 16 is generally only used at low speeds. Twisting joystick30 in the directions of arrow T turns nozzle 18, steering boat 10 to theleft or to the right. Centering forces (or centering torque, in the caseof rotation) provided, e.g., by springs, bias joystick 30 to its neutralpositions. The structure, operation, and electrical circuitry ofjoystick 30 are described in detail in U.S. patent application Ser. No.09/146,596, entitled “Stick Control System for Waterjet Boats,” filedSep. 3, 1998, and incorporated herein by reference in its entirety.

[0038] Referring to FIG. 4B, an autopilot 32 includes a compass 34 andelectrical circuitry 36. When autopilot 32 is engaged, it acts tomaintain the course of boat 10 in the direction of the current readingof compass 34. Autopilot 32 can be, e.g., a Robertson autopilot, such asthe Robertson AP20, with modified software and circuitry, as describedbelow with reference to FIG. 5.

[0039] At a given moment, nozzle 18 is controlled by either joystick 30or autopilot 32, but not both. Autopilot 32 controls nozzle 18 wheneverjoystick 32 is in its neutral, “un-torqued” position, and joystick 30controls nozzle 18 whenever nozzle 18 is twisted by an operator.

[0040]FIG. 5 schematically illustrates communication between joystick 30and the modified Robertson autopilot 32. FIG. 5 is divided into twosides: the joystick circuitry 50 and the autopilot circuitry 52.Joystick circuitry 50 includes control circuit 54, a joystick circuitinterface 56, and a NEMA translator 58. (“NEMA” stands for NationalElectrical Marine Association. NEMA is a uniform wiring and data codestandard.) NEMA translator 58 translates NEMA command sentences receivedfrom autopilot 32 into the language of control circuit 54, and alsotranslates commands issued by control circuit 54 into NEMA. Joystickcontrol circuit 54 connects to joystick 30 via a translator 59.Translator 59 translates movement of joystick 30 into electricalcommands understood by control circuit 54.

[0041] Joystick circuitry 50 is located on two printed circuit boardswithin a single electronics enclosure. Control circuit 54 is located ona main printed circuit board, and interface 56 and translator 58 arelocated on an auxiliary board. Alternatively, interface 56 andtranslator 58 can be integrated onto the main board. The structure andoperation of control circuit 54 and the main printed circuit board isdescribed in U.S. application Ser. No. 09/146,596.

[0042] Autopilot circuitry 52 includes an autopilot interface 60 and aNEMA translator 62. Autopilot circuitry 52 is located on a circuit boardwithin Robertson autopilot 32.

[0043] Joystick circuity 50 connects to autopilot circuity 52 via twoNEMA cables 64 a, 64 b. NEMA cables 64 a, 64 b transmit NEMA commandsentences between translator 58 and translator 62. Control circuit 54and autopilot 32 also separately connect by electronic cabling 66 a, 66b to a hydraulic steering pump 68, which steers the nozzle.

[0044] The manner in which control circuit 54 and autopilot 32 negotiatecontrol over pump 68 is described below.

[0045] Steering a Boat Using the Joystick and Integrated Autopilot

[0046] A boat 10 having integrated joystick 30 and autopilot 32 can becontrolled as follows. First, an operator turns on the boat'selectronics and starts the boat's engine. The operator then placesjoystick 30 in “docking mode” by choosing docking mode on the modeselection switchpanel (not shown), and engages waterjet drive 12. (Thedifferent operating modes for joystick 30 and the mode selectionswitchpanel are described in U.S. patent application Ser. No.09/146,596.) When drive 12 is first engaged, bucket 14 is in its neutralposition, so that drive 12 does not immediately cause boat 10 to moveforward or backward.

[0047] Next, the operator turns on autopilot 32 by activating autopilotpower switch 37. (Alternatively, autopilot power switch 37 can be lefton, so that turning on the boat's electronics automatically powersautopilot 32.) Since joystick 30 is in its neutral position when powerswitch 37 is activated, autopilot 32 immediately engages, andimmediately acts to keep the bow of the boat steady. The operator thenreleases boat 10 from its dock line. Autopilot 32 continues to keep thebow of the boat from drifting while the operator releases the dock line,and while the boat remains still in its slip (while bucket 14 remains ina neutral position).

[0048] After releasing boat 10 from its dock, the operator centers theboat within its slip by engaging bowthruster 16. Engaging bowthruster 16at very low speeds allows direct sideways maneuvering of boat 10, asdescribed below. Once the boat is centered, the operator uses joystick30 to lower bucket 14, causing boat 10 to move out of its slip.

[0049] After leaving the slip, the operator can change the boat'sheading by twisting joystick 30. When the operator twists joystick 30,translator 59 translates the twisting movement into an electricalcommand and sends it to control circuit 54. Control circuit 54 thenissues a command sentence instructing autopilot 32 to release control ofsteering pump 68. The command sentence issued by control circuit 54travels through interface 56 to translator 58, where it is translatedinto NEMA. The command then travels over NEMA cable 64 a to translator62, which translates the command into language understood by autopilot32.

[0050] When autopilot 32 receives the command via interface 60, it sendsan acknowledgement sentence back toward control circuit 54. Theacknowledgement sentence travels through interface 60, is translatedinto NEMA by translator 62, and travels over cable 64 b to translator58. Translator 58 then translates the acknowledgement into languageunderstood by control circuit 54. Control circuit 54 then receives theacknowledgement via interface 56, and takes control of hydraulicsteering pump 68. Joystick 30 now controls movement of hydraulicsteering pump 68 and nozzle 18.

[0051] Once the operator has adjusted the course of boat 10 to a newdesired heading, he or she releases joystick 30, and the centeringtorque returns joystick 30 to its neutral, “un-torqued” position. Asjoystick 30 returns to its neutral position, nozzle 18 returns to itscentered position (shown in FIGS. 3B and 3E).

[0052] The centering movement of joystick 30 is translated by translator59 into an electrical signal, and sent to control circuit 54. After apredetermined delay, e.g., about 1.5 seconds (long enough to allownozzle 18 to re-center), control circuit 54 sends a command to autopilot32 to resume control of steering pump 68. The command sentence travelsto autopilot 32 in the manner described above. When autopilot 32receives the command, it retakes control of steering pump 68, and sendsan acknowledgement sentence back to control circuit 54. Autopilot 32then maintains the current heading of boat 10 until the operator againtwists the nozzle.

[0053] At any time, the operator can adjust the speed of boat 10 byraising or lowering bucket 14 using joystick 30. Since bucket 14 is notintegrated with autopilot 32, the operator can adjust the speed withoutinterfering with the autopilot-based steering. Autopilot 32 also acts tokeep the bow of the boat pointed in a desired direction when bucket 14is in the position shown in FIG. 2C, and boat 10 is moving in reverse.

[0054] The autopilot-based steering method can be used throughout theboat's journey, from the moment autopilot power switch 37 is activateduntil after boat 10 has been re-secured to its dock. The autopilot'spower need not be deactivated until after the boat has been re-securedto its dock line.

[0055] The operator can use the above described steering method at highspeed, low speed, and very low speed, e.g., when maneuvering or dockingthe boat. To facilitate use of the integrated joystick/autopilotsteering method at a variety of speeds, the response sensitivity ofautopilot 32 varies depending on the speed of boat 10.

[0056] Response sensitivity of an autopilot is measured by its“P-factor,” where the P-factor equals the number of degrees the nozzlewill rotate to correct for a one degree error in course heading. Forexample, if compass 34 in autopilot 32 senses that the boat's heading isoff by 2°, and the P factor is 3, then autopilot 32 will cause nozzle 18to rotate 6°. A standard Robertson autopilot has a programmable P factorthat shifts between a low-speed P factor and a high-speed P factor basedon input from a boat speed sensor; the low and high-speed P factors canbe adjusted within a range of 0 to 4.

[0057] The modified Robertson autopilot 32 has an extended P-factorrange, e.g., from 0 to about 7, and the P-factor varies depending on thespeed of the boat. In a preferred embodiment, autopilot 32 operates atone of three different predetermined P-factor response modes. When boat10 is moving at high speed (forward speed greater than, e.g., about 8knots), autopilot 32 operates in “high speed mode,” and the P factor is,e.g., about 2; when boat 10 is moving at low speed (forward speed of,e.g., about 2 to 8 knots), autopilot 32 operates in “low speed mode,”and the P factor is, e.g., about 4; and when boat 10 is moving at a verylow speed, e.g., 4, 3, or 2 knots, autopilot 32 operates in “maneuveringmode,” and the P-factor is generally greater than 4, e.g., about 5, 6,or 7.

[0058] Maneuvering mode is typically used when docking a boat,maneuvering a boat within its slip, or maneuvering a boat through aseries of close obstacles. Maneuvering mode is triggered by activatingbowthruster 16 with sideways movement of joystick 30 (in the directionof arrows L or R in FIG. 4A). When bowthruster 16 is released, theresponse mode changes from maneuvering mode back to low speed mode aftera predetermined delay of, e.g., about 1.5 seconds.

[0059] Alternatively, joystick 30 and autopilot 32 can have greater orless than three possible P-factors, or can have a sliding P-factor scaledirectly correlated to the speed of boat 10.

[0060] Maneuvering a Waterjet Boat in Maneuvering Mode

[0061] The highly sensitive maneuvering mode is most useful in waterjetboats. As described above in the Background, steering a waterjet boat,particularly at docking speeds, can be difficult. In prior art boats, anoperator would have to simultaneously control the bowthruster, bucket,and nozzle to achieve precision movements, such as direct sidewaysmovement of the boat. By contrast, using the autopilot-based maneuveringmode, an operator can allow the autopilot to keep the bow pointed in adesired direction, simplifying steering.

[0062] In maneuvering a boat using bowthruster 16 and autopilot 32,autopilot 32 essentially “chases” the bow. To maneuver boat 10 using theautopilot-based maneuvering mode, an operator first points the bow ofthe boat in a desired direction by twisting joystick 30, as describedabove. Next, the operator engages bowthruster 16, shifting the boat tomaneuvering mode, and causing the bow of the boat to move sideways. Whenthe bow of boat 10 shifts in response to activation of bowthruster 16,autopilot turns nozzle 18 to compensate, so that the bow of boat 10continues to point in the desired direction. Autopilot 32, therefore,“chases” the bow, facilitating direct sideways movement of boat 10.

[0063] Sideways movements can be combined with forward or reversemovements, as forward or reverse movement of the joystick will produce acorresponding movement of the boat. In short, with the autopilot-basedmaneuvering system activated, the boat will move in the direction thatthe operator points the stick, while maintaining the current heading.Should a slight heading adjustment be desired, the operator simplytwists the joystick to achieve the new heading, and then continues topoint the stick in the direction desired.

[0064] The autopilot-based, very low speed maneuvering aspect of theinvention is preferably integrated with the autopilot-based steeringmethod described above. That is, autopilot 32 remains engaged at high,low, and maneuvering speeds unless the operator is actively twistingjoystick 30. The autopilot-based maneuvering, however, need not beintegrated with autopilot-based steering; a waterjet boat that does nothave a joystick and does not employ the autopilot-based steering systemdescribed above can still employ autopilot-based maneuvering.

[0065] For example, referring to FIG. 6, a waterjet boat 110 includes anautopilot 132 for low speed maneuvering. Autopilot 132 has a P-factorof, e.g., about 7, and is activated and deactivated by manually pushinga button 134, rather than by releasing a joystick. When autopilot 132 isactivated, it keeps the bow of boat 110 pointed in a desired direction,as described above. Autopilot 132 also includes a steering knob 136. Theheading of waterjet boat 110 can be adjusted slightly by turning knob136.

[0066] To maneuver boat 110 using autopilot 132, an operator firstreduces boat 110's speed to, e.g., one knot, and points the bow of boat110 in a desired direction. The operator then activates autopilot 132 bypushing button 134, engaging the bucket and bowthruster as needed tomaneuver boat 110. If the operator decides to adjust boat 110's heading(adjust the direction the bow is pointing), the operator can turn knob136.

[0067] Other Embodiments

[0068] Other embodiments are within the scope of the claims. Forexample, bowthruster 16 can be integrated into the autopilot-basedsteering method. Autopilot 32 can be designed to control bothbowthruster 16 and nozzle 18 to maintain a heading at low speed.Movement of joystick 30 to engage either nozzle 18 or bowthruster 16would reclaim control from autopilot 32.

[0069] The autopilot-based steering method can be used with steeringsystems that employ a control device other than a joystick stick controlmember. And when a stick control member is used, movements other thantwisting could be what causes the autopilot to disengage. For example,if the waterjet nozzle is controlled by sideward movement of a joystickrather than by twisting, the autopilot could be automatically disengagedon sensing sideward movement.

[0070] The invention described above is particularly useful for smallwaterjet boats (boats less than 75 feet long), but could also be used inlarger waterjet boats.

[0071] The autopilot-based steering method of the invention can be usedin boats other than waterjet boats. For example, in propeller basedboats, an autopilot can be designed to control the boat's course unlessan operator is currently commanding a change in course.

What is claimed is:
 1. A waterjet boat in which forward and reversepropulsion is provided by one or more jets of water directed generallylongitudinally, the boat comprising: a steering system including anozzle capable of rotation about a generally vertical axis fordeflecting the jet to impart a side component of force to the boat; arotational thrust system that tends to rotate the boat about a verticalaxis and to produce a sideward movement of the bow of the boat; ajoystick device for use by the operator of the boat for manual controlof the steering system; and an autopilot configured to be engaged whenthe boat is moving at a very low rate of speed (less than about 4 knots)and that controls the steering system to maintain the bow of the boatpointed in a desired direction, wherein a movement of the joystickdevice activates the rotational thrust system, thereby producing acorresponding sideward movement of the bow of the boat, and theautopilot automatically causes a movement of the nozzle to return thebow to the desired direction, thereby producing an overall sidewardmovement of the boat, wherein the rotational thrust system comprises abow thruster, wherein the autopilot has a P factor and the autopilot isconfigured to operate at a P factor greater than 4 at the very low rateof speed, and wherein the rotational thrust system is controlled by afirst movement of the joystick device, wherein the nozzle is controlledby a second movement of the joystick device, wherein forward and aftthrust of the waterjet is controlled by a third movement of the joystickdevice, wherein the joystick device has a stick control member, and thefirst movement is sideward movement of the stick control member, thesecond movement is rotation of the stick control member, and the thirdmovement is forward and aft movement of the stick control member, andwherein the boat is less than 75 feet in length.
 2. The boat of claim 1,wherein the autopilot is configured to operate at a P factor greaterthan 6 at the very low rate of speed.
 3. The method of claim 1, whereinthe very low rate of speed is than about 2 knots.
 4. The boat of claim1, wherein the joystick device has a stick control member that is biasedto a neutral position by a centering force.
 5. The boat of claim 4wherein the joystick device has a stick control member capable ofrotation and with a neutral zero rotation position, and the stickcontrol member is biased by a centering torque such that it returns toits neutral position when released by the operator.
 6. The boat of claim5 wherein the centering torque that biases the stick control member toits neutral position is provided by a spring.
 7. The boat of claim 1wherein the autopilot has a P factor, and the autopilot operates at alower P factor when the boat is traveling at a higher speed than whenthe boat is traveling at a lower speed.
 8. The boat of claim 1 whereinthe joystick device has a stick control member and the stick controlmember is used to steer the boat at higher speeds.
 9. The boat of claim8 wherein at higher speeds the steering system steers the boat towardsport or starboard when the stick control member is displaced from itsneutral position.
 10. The boat of claim 9 wherein the operator isactively commanding the stick control member to change the boat's coursewhen the operator displaces the stick control member from its neutralposition.
 11. The boat of claim 5 wherein the autopilot is engagedwhenever the stick control member is in its neutral position.